Dana,
My hat's off to Shell Valley. Any manufacturer that scans these threads, and jumps in to offer assistance to a builder of one of their cars running into a snag deserves a pat on the back. Your efforts did not go unnoticed. Thanks!
The 302 & 351W use the same motor mounts. Where it gets tricky is that the 351W is taller and wider than the 302. The 351W then may require a slightly lower mounting height in the frame so the air cleaner clears the hood, yet provides sufficient ground clearance for the
oil pan. The trans may need to be dropped a like amount or the sidepipes may not be level. We're only looking at 3/4" - 1" lower on the 351W.
Some manufacturers use different frame and trans mounts specific for the 302 and 351W or some shimming method to compensate for the install height difference.